1. Technical Field of the Invention
The invention relates to the field of charging for services in a wireless communications system and in particular to a method of controlling charging procedures for message services which enable transport of messages from a mobile station to a service center. The invention further relates to a network node which implements such a control method and a wireless communications system comprising such a network node.
2. Description of the Prior Art
Wireless communications systems (WCS) like the “Global System For Mobile communications” (GSM) not only provide speech services but also data services like “Short Messages Services” (SMS). SMS enables the transmission of alphanumerical short messages of up to 160 ASCII characters.
GSM distinguishes between the “Mobile Terminating Short Message Service” (SMS-MT), for the reception of short messages, and the “Mobile Originating Short Message Service” (SMS-MO), enabling a GSM user to send such a message for instance to another GSM user, a user of a different telecommunications system, or an electronic mailbox. For enabling “Point-to-Point Short Message Services” (SMS-PP), GSM is in contact with a “Short Message Service Center” (SM-SC or simply service center, SC). A single SC may be connected to several GSM networks. In each of these GSM networks, one or several functional entities are in charge of interfacing the SC. In GSM, two types of such entities are defined: The “SMS-Gateway Mobile Services Switching Center” (SMS-GMSC) for SMS-MT and the “SMS-Inter Working Mobile Services Switching Center” (SMS-IWMSC) for SMS-MO. Both entities are called SMS-Gateway for short.
The communication of a WCS in the form of a GSM network with a SC for the case of SMS-MO is depicted in FIG. 1. As can be seen from FIG. 1, the GSM network comprises a mobile station (MS) which communicates with a “Mobile services Switching Center” (MSC). A “Visitors Location Register” (VLR) may be linked to one or more MSCs and is in charge of temporarily storing subscription data for all GSM users currently situated in the service area of MSC/VLR. The main function of MSC/VLR is to coordinate the setting-up of a connection from MS to the GSM network. MSC/VLR communicates with a SMS-IWMSC which provides a fixed GSM gateway for interconnection, or “Inter Working” (IW), with SC.
Communication between the individual components depicted in FIG. 1 is performed by using stacks of individual communication protocols. In the GSM standard, the stack of protocols between SMS-Gateways and SC is left open. What is specified in the GMS standard is a set of communication protocols for conveying the short messages between MS and SMS-Gateway.
The transportation of short messages within the GMS network involves essentially two segments: The MS to MSC/VLR segment, and the segment between MSC/VLR and SMS-Gateway. The “Short Message Transport Protocol” (SM-TP) specified between MS and SC is an end-to-end protocol including some features of an application protocol. Between MS and SMS-Gateway, the latter being the point of interconnection of SC in GSM, the mobile application part (MAP) protocol MAP/H interconnects with the “Short Message Relay Protocol” (SM-RP), which itself relays on the “Short Message Control Protocol” (SM-CP) for transport between MS and MSC/VLR.
The general architecture of protocols for the transport of point-to-point short messages is specified in TS GSM 03.40, where an example of a protocol stack for the connection of an SC to an MSC/VLR can be found. TS GSM 04.11 specifies both the SM-RP and the SM-CP protocols, which are used to carry short messages between MS and MSC/VLR. The MAP/H protocol is specified in MAP, that is to say by TS GSM 09.02.
In the following, the steps when sending a SMS-MO will be explained in more detail with reference to FIG. 1.
In a first step, symbolized by arrow 1, MS sends a SMS to MSC/VLR (CP-DATA Direct Transfer Application Part (DTAP) message). In a second step, symbolized by arrow 2, MSC/VLR answers to MS and acknowledges receipt of SMS (CP-DATA-ACK DTAP message). In a third step, symbolized by arrow 3, SMS is routed from MSC/VLR via SMS-IWMSC, i.e. the SMS-Gateway, to SC of terminating MS (ForwardShortMessage/ForwardMOShortMessage MAP messages). In a fourth step, symbolized by arrow 4, MSC/VLR receives an acknowledgment from SC via SMS-IWMSC about the result (ForwardShortMessage/ForwardMOShortMessage result messages). For example, this acknowledgment may be a confirmation that SMS has actually been received by SC. In a fifth step, symbolized by arrow 5, MSC/VLR forwards the acknowledgment received from SC to MS (CP-DATA DATAP message). In a sixth step, symbolized by arrow 6, MS acknowledges receipt of the CP-DATA DTAP message symbolized by arrow 5 (CP-DATA-ACK DTAP message). Upon receipt of the CP-DATA-ACK DTAP message, MSC/VLR accesses a charging unit CU, as indicated by arrow 7, and initiates charging by writing to a call detail record. In step 8, symbolized by arrow 8, MSC/VLR clears the transaction (Clear Command DTAP message).
If MS is intentionally or unintentionally made not reachable between the fourth and the fifth step, SMS has been successfully sent to SC, which forwarded SMS for example to a terminating MS. However, charging is generally not initiated before originating MS is informed about the result, i.e. before MSC/VRL signals to MS that SMS was successfully sent to SC (arrow 5) and that this transaction will now be charged. In such a case, i.e. when MS cannot be informed about the successful sending of a SMS towards SC, many service providers will not charge the SMS at all to avoid possible overcharging. Consequently, an SMS can be sent free of charge by intentionally or unintentionally making the originating MS not reachable shortly after sending of an SMS.
There is, therefore, a need for a method of controlling charging procedures for message services which renders the charging more secure and more robust against fraud. There is also a need for a switching center which implements such a method and a wireless communications system comprising such a switching center.